Ternary Ga1-xAlx.As is changed into quaternary Ga1-xAlxAs1-yPy compounds by two liquid phase epitaxial techniques in making GaAs-Ga1-xAlx As double-heterostructure lasers. The results measured by x-ray diffractometer show optimal GaAs-Ga1-xAlxAs1-yPy heterostructure lattice mismatching is decreased down to 1x10-5, the relative mismatch stress is 1.4X107 dynes/cm2. Observation with scanning electron microscope shows that the heterostructure interface is very flat. This method can significantly improve the heterostructure quality. Double heterostructure semiconductor lasers with efficient, long lifetime can be expected.